STORY: The agreements, described by Britain as "a step-change in defense and security cooperation", will go some way to assuage fears in Finland and Sweden over threats made by Russia should either nation decide to join NATO.
"It's worth emphasizing that if Sweden were attacked and looked to us for help and support, then we would provide it," Johnson said after meeting Swedish Prime Minister Magdalena Andersson.
Johnson would not say whether the UK would send troops to Sweden in the event of an attack.
"It is up to Sweden to make the request and to spell out exactly what support is requested," he said.
Sweden is coordinating with Finland over possible NATO membership and both countries are expected to announce their intentions in the coming days.